1. Field of Invention
The present invention relates generally to converters and, more particularly, to converters having integrated active clamp and primary bias circuits.
2. Description of the Prior Art
A DC-DC converter, such as an SMPS (Switch Mode Power Supply) converter, is a device that converts a DC input voltage to a different DC output voltage.
SMPS converters are very versatile. They can be used, for example, to:                1. step down an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Buck Converter or Step-Down SMPS,        2. step up an unregulated dc input voltage to produce a regulated dc output voltage using a circuit known as Boost converter or Step-Up SMPS,        3. step up or step down an unregulated dc input voltage to produce a regulated dc output voltage using a Buck-Boost Converter.        
Converters are also characterized as either flyback converters, forward converters or combinations thereof.
A flyback converter generally has a Buck-Boost SMPS topology in which, during the first half of the switching period, energy is stored in the transformer and during the second half of the period this energy is transferred to the transformer secondary and into the load.
A forward converter generally has a Buck-derived SMPS topology in which energy is transferred to the transformer secondary winding and the load when the switching transistor is on.
Whether a converter is a flyback converter, a forward converter or a combined forward-flyback converter, it is necessary to provide a primary bias circuit for the controller which controls the switching cycle and also to provide a primary clamp circuit.
In the case of forward converters, the purpose of the clamp circuit is to reset the core flux of the transformer. More specifically, in a forward converter only positive voltage is supplied across the transformer core during the conduction of the primary switch. As a result, the flux continues to increase with the application of the input voltage. The flux increases until the core saturates and circuit failure occurs. Accordingly, it is necessary to supply negative voltage to reset the core flux. This is generally effected with a clamp circuit.
In the case of flyback transformers used in flyback converters, although they do not require flux reset circuitry, they do require clamp circuitry to clamp any voltage spike on the primary power switch.
There are several types of clamp circuits employed which generally fall into two categories: active and passive. Active clamp circuits are generally preferred. This is particularly so in the case of flyback converters when a high voltage is used. This is because in this case there is considerable insulation material between the primary and the secondary of the transformer which leads to large leakage inductance. This, in turn, results in large voltage spikes on the primary which require an active clamp circuit for effective clamping.